In a typical unbalanced (differential) hydraulic cylinder, the cross-sectional area of the chamber on the head side of the piston is greater than the cross-sectional area of the chamber on the rod side of the piston. When the cylinder is extended, more fluid is needed to fill the head-end or extend chamber of the cylinder than is being discharged from the rod-end or retract chamber. Conversely, less fluid is needed to fill the rod-end chamber than is being discharged from the head-end chamber when the cylinder is being retracted.
In modern machinery using electro-hydrostatic actuation (EHA) systems it may by advantageous to locate the electric motor driven pumps and hydraulic actuators in areas remote from the tank or reservoir. This distance increases the likelihood of cavitation and associated pitting occurring in the hydraulic pumps and associated control valves as the hydraulic fluid is exposed to sharp and rapid pressure drops resulting from the demands of highly responsive actuators. To prevent vacuum and associated cavitation in the lines, pumps and valves leading to the inlet side of the actuation system pumps, it is desirable to provide and maintain an elevated pressure in the hydraulic passages leading from the tank or reservoir to the actuation system pump inlet. This is accomplished in prior art by the installation of one or more pressurized accumulators in a closed hydraulic circuit and in communication with the inlet or low pressure passages leading to each pump of the EHA system(s) and thereby maintaining adequate hydraulic fluid pressure during all actuation activities. The pressurized accumulator is typically of a bladder type having a gas pressure charged volume separated from the hydraulic fluid by a flexible membrane or bladder or alternately of a metal bellows or spring loaded piston type.
As the result of the addition of a pressurized accumulator in closed circuit communication with the EHA system, several disadvantages are incurred. The amount of hydraulic fluid in the accumulator must exceed that which is rejected by all contracted cylinders in closed circuit with it by allowances for thermal expansion and contraction of all of the hydraulic fluid in the system, hydraulic fluid leakage and the included volume of the gas chamber. As a result, the physical size and weight of the accumulator is undesirably large. Also, since some of the hydraulic fluid contained in the accumulator is not circulated to and from a tank or reservoir that is open to the atmosphere, entrained air bubbles are not allowed to escape from the accumulator. This problem may be compounded if gas leakage should occur across the accumulator bladder. Also, gas charged accumulators require added maintenance due to the need for a gas charging means. There is also the threat of external nuisance gas and hydraulic fluid leakage during storage since at least a part of the system remains under pressure at all times.
An exemplary prior art system for controlling an unbalanced hydraulic cylinder 20 is illustrated at 21 in FIG. 1. The system 21 provides for flow management between a two port pump 23 and the unbalanced hydraulic cylinder 20. The pump 23 is of a bi-directional type that is continuously driven in one direction by an electric motor or other drive means. The pump has one inlet/outlet port 26 connected by a line 27 to the extend chamber 28 of the hydraulic cylinder 20 and the other inlet/outlet port 30 connected by a line 31 to the retract chamber 32 of the hydraulic cylinder. The displacement of the pump is controlled by a control valve 35, which in the case of a piston-type pump controls the tilt of the swash plate that in turn controls the flow direction and displacement of the pump. The position of the control valve 35 is determined by a directional valve 36 that selectively connects the outlet 37 of a charge pump 38 via line 40 to either side of the control valve and the opposite side to a system tank or reservoir 41 via line 42. The charge pump 38 is continuously driven at the same speed and in the same direction as the pump 23. Much of the output of the charge pump is dumped across a relief valve 44, with consequent heat generation and energy loss.
For flow management of the unbalanced hydraulic cylinder 20, the lines 27 and 31 are connected by respective pilot-operated check valves 46 and 47 to a common line 48 connected between the outlet 37 of the charge pump 38 and an accumulator 50. In this type of pump, both the accumulator and charge pump are needed to support supply pressure and flow requirements. The accumulator supports the charge pump to keep the inlet pressure to the pump 23 at an elevated level during high accumulator demands to avoid cavitation during fast acceleration of the pump. The pressure on common line 48 is determined by the accumulator or an adjustable pressure relief valve 44 connected between the common line 48 and the tank 41. The adjustable pressure relief valve 44 or accumulator 50 also determines the pressure supplied to the directional valve 36 for operating the control valve 35. The illustrated prior art system further includes adjustable pressure relief valves 52 and 53 respectively connecting lines 27 and 31 to the common line. The pressure relief valves 52 and 53 protect the pump and cylinder from the possibility of over pressurization in the event that an excessive external overload on the cylinder should be applied when the pump is in a neutral position preventing relief of a high pressure in line 27 or 31.
In operation, the valve 36 may be controlled to cause the pump 23 to supply hydraulic fluid to the line 27 for extending the hydraulic cylinder 20. Flow leaving the hydraulic cylinder will flow to back to the pump. Because of the cylinder unbalance, such flow will be less than the volume of flow being supplied to the extend side of the cylinder. This will cause the pressure on line 31 to drop below the pressure on common line 48, whereupon make-up flow can be provided from the accumulator 50 and/or from the tank 41 via the charge pump 38. At this time, pressure supplied by pilot line 54 from line 27 will have caused the pilot-operated check valve 47 to have opened.
When the pump 23 is operated in the reverse direction, there will be an excess volume of fluid leaving the cylinder 20. This excess flow will be diverted to the common line 48 by the pilot-operated check valve 46 that will then be open by pilot pressure supplied from the line 31 via pilot line 56.
FIG. 2 shows another prior art system 60 that uses two bidirectional pumps 61 and 62 and a piston-type variable pressure accumulator 63. The accumulator pressure can be raised or lowered by an electrically powered actuator 64 to increase control flexibility. An elevated pressure would be used, for instance, for normal electro-hydraulic actuator (EHA) operation. A lowered pressure might be used when retracting the cylinder 66. The system also includes a pump 68 that is continuously driven by an engine, electric motor, or the like. A switching valve 69 either supplies hydraulic fluid from the pump 68 to replenish leakage and charge the accumulator 63 or re-circulates hydraulic fluid back to the tank (reservoir) 71 with an associated heat loss. Reference may be had to U.S. Pat. No. 6,962,050 for further details of an exemplary system of the type shown in FIG. 2.
FIG. 3 shows still another prior art system 90 using closed circuit flow management. The system 90 utilizes what is commonly referred to as a three-port pump 91, such as shown in U.S. Pat. Nos. 5,144,801 and 6,912,849. The three-port pump is designed such that an internal porting arrangement within the pump provides a division of flow in proportion to the cylinder head end and cylinder rod side annular areas. When the cylinder 94 is extending, for example, the volumetric output of the pump flowing into the cylinder head end 95 at an elevated pressure is equal to the sum of hydraulic fluid taken into the pump at a reduced pressure from the cylinder rod side 96 plus the necessary make up hydraulic fluid provided by a low pressure accumulator 97. Conversely, when the cylinder is retracting, the volumetric flow at a reduced pressure flowing from the cylinder head end 95 and into the pump 91 is equal to the sum of hydraulic fluid at an increased pressure flowing to the cylinder rod end side 96 plus an excess of hydraulic fluid expelled into the low pressure accumulator 97.
In excavating equipment and other working machines, large liquid-cooled motors have been used to drive the pumps used to hydraulically power the functional cylinders. Accordingly, a liquid cooling system heretofore has been needed to maintain the operating temperature of the motors and associated electronic power modules at an acceptable operating temperature. The flow management and temperature control systems heretofore employed have been inefficient, expensive and/or complicated.